In MacDonald et al. U.S. Pat. No. 5,136,413 issued Aug. 14, 1992, entitled IMAGING AND ILLUMINATION SYSTEM WITH ASPHERIZATION AND ABERRATION CORRECTION BY PHASE STEPS, an imaging system is disclosed which uses phase plates. This reference is assigned to the assignee hereof and is incorporated into this disclosure by reference.
This MacDonald et al. imaging system has the aspherization and aberration correction in the original imaging system design corrected by the insertion of two phase plates, these phase plates each having differing distances from the lenses of the imaging system. The first phase plate is an aplanatism plate which acts as an asphere to place each light ray within the correct position on the second phase plate to satisfy the so-called "sine" condition. This sine condition is satisfied when the ratio of the sine of the angles made between corresponding rays at the object and image points and the axis of the optical system is constant. The second phase plate is an axial stigmatism plate which ensures that each ray with the beam focuses at the focal point.
While the subsequent discussion is confined to lens or refractive imaging systems, it is to be understood that the methods and techniques described herein apply word for word to reflective and catadioptric imaging systems.
The entire MacDonald et al. disclosure is directed to the simplified construction of a lens system so that the aspherization and aberration correction do not have to be added to the surfaces of the optical elements of the system. A lens system built with the disclosed phase plates has vastly simplified lens surface configuration.
Unfortunately, lens systems commonly contain both individual optic fabrication errors as well as mounting errors. Systematic errors, generally a result of design errors, are also not uncommon. As a result of these various errors a system will normally include one or more primary image defects, usually referred to as the Seidel Aberrations. Such aberrations can include distortion, curvature of field, spherical aberration, coma, and astigmatism. Only a system which is theoretically perfect in design, fabrication, and assembly will be free of these effects. This theoretically perfect optical system is sometimes referred in the art as being "diffraction limited." It is only the diffractive properties of the light passing through the lens that limit the lens system optical performance.
The present invention is aimed at correction of existing lens system errors so that they may more closely approach this diffraction limit. Since lens systems fail to approach the diffraction limit because of individual optic design, fabrication, and mounting errors as well as systematic errors, the phase plates used in the following specification will be different for each lens system corrected.
The disclosed invention has at least two distinct useful areas. First, existing lens systems can be modified. For example, on existing steppers utilized in the construction of micro chips and circuits, the original optical resolution can be improved to achieve the required higher resolution for new (smaller) generations of chips and circuits thereby avoiding the considerable cost of stepper replacement. In addition, laser machining tools utilizing projection imaging can be improved to have better resolution. Second, new lens systems can be fabricated more economically utilizing the disclosed technique. For example, stepper lens systems can be designed with intervals within the lens system to receive the specially designed plates of this invention. With such a lens system, the original costs of producing the stepper lens system can be reduced.
Having noted that this invention finds utility both with new and existing lens systems, the difference between the invention herein and that of the MacDonald et al. disclosure must be understood. Following the teaching of MacDonald, similar phase plates for similar lens systems will be utilized. Following the teaching of the present disclosure will theoretically yield different phase plates for similar lens systems to correct the inevitably different and random errors of lens system construction.
Having established this distinction, and before reading the following specification, some attention to terminology is useful.
This invention is primarily directed toward imaging systems, although minor variations in the technique would make it apply to other optical applications. In order to measure the imaging errors in conventional optics, an image must be projected. This image is projected by the individual "object points" of an actual or real object. The object is precisely registered relative to the lens system under test. For the purposes of this discussion, consider an object with regularly repeating points that may be easily correlated to the corresponding points on the produced image of the object. Further, and to describe the image of the object points, the terms "ideal image points" and "ideal image" are used. The ideal image is that image which would be projected on the image plane from the object if the lens system were perfect or ideal.
In order to correct the optical system in accordance with this invention, it is useful to compute a new set of desired positions near the real object based on the ideal image. The object points located in these desired positions are referred to as the apparent or desired object points and they comprise the apparent or desired object. Further, and with the interferometric testing of the optics herein, it will be desired to have rays of certain angularity be displaced. This desired displacement will be referred to as "desired ray trajectory." It will be noted that this desired ray trajectory may or may not pass through either of its corresponding object positions. As will be made clear hereafter, it is the function of the corrector plates of this invention to make rays originating at or near the actual object, appear to be originating at the apparent object, thereby resulting in the optical system approaching diffraction limited performance. All this will become more clear in reading the following Summary of the Invention.